1. Field of the Invention
The present invention relates to heat-sensitive adhesive materials which can be applied to a variety of adherends such as resinous adherends, are typically useful in the field of point-of-sale (POS) systems in food industry, do not require a release paper (liner), are non-adhesive before application (thermal activation) but become adhesive upon thermal activation and are suitable as liner-less labels. It also relates to an article adhered with the heat-sensitive adhesive material, and a process and apparatus for efficiently thermally activating the heat-sensitive adhesive material.
2. Description of the Related Art
Adhesive sheets have been increasingly used in labels such as labels for indicating price, trade description (bar codes), quality, measure or advertisement (stickers). Conventional adhesive sheets each with a release paper and an adhesive layer laminated on the side opposite to the information recording side of such labels have been widely used, since these adhesive sheets can be easily applied to an adherend only by removing the release paper and pressing the adhesive sheets onto the adherend.
The release papers of these conventional adhesive sheets must be removed before use, which complicates application procedures. The release papers removed before use are not recycled and are disposed in most cases. The adhesive layer (pressure-sensitive adhesive layer) has tackiness or adhesiveness and is very difficult to handle after removal of the release paper. For example, when an adhesive sheet is placed on an adhered accidentally and an attempt is made to remove the adhesive sheet in order to apply the same properly, the adhesive sheet becomes curled or wrinkled and breaks at worse.
As a promising candidate for solving the problems, heat-sensitive adhesive sheets that do not exhibit adhesiveness at room temperature and do not require release papers have received attention. Such heat-sensitive adhesives comprise, for example, a solid plasticizer and a thermoplastic resin emulsion as essential components and may contain a tackifier and other additives. This type of heat-sensitive adhesives is applied to a side of a substrate opposite to a printing or recording surface and thereby yields a heat-sensitive adhesive material. The surface of an adhesive layer of the heat-sensitive adhesive material exhibits no adhesiveness or tackiness at room temperature, but is capable of exhibiting adhesiveness when heated and keeps its adhesiveness for a while after heating. Certain heat-sensitive adhesive materials comprise a liquid plasticizer instead of the solid plasticizer. The liquid plasticizer is generally encapsulated in microcapsules, and the shell (microcapsules) breaks or allows the liquid plasticizer to pass therethrough by heating, and the liquid plasticizer dissolves the thermoplastic resin emulsion and the tackifier to thereby exhibit adhesiveness, as in the solid plasticizer. These heat-sensitive adhesive materials containing a solid plasticizer or a liquid plasticizer do not require release papers and are advantageous in resource saving and environmental protection, in contrast to the conventional adhesive materials.
However, such conventional heat-sensitive adhesive materials have the following problems.
Plastic wrap for use in, for example, food in point-of-sale (POS) systems has been changed from poly(vinyl chloride) wrap to polyolefinic wrap to avoid dioxins and other environmental issues. POS labels are applied to such polyolefinic wrap, but exhibit much lower adhesiveness to and wettability with the polyolefinic wrap than the poly(vinyl chloride) wrap and are often delaminated.
In addition, the heat-sensitive adhesive materials have insufficient adhesiveness to polyolefinic wrap. This is because the polyolefinic wrap includes less additives such as plasticizers and stabilizers and thereby exhibits less wettability than the poly(vinyl chloride) wrap. These heat-sensitive adhesive materials are also used in labels for envelopes made of a nonwoven fabric of polyethylene or another polyolefin. In this case, they also exhibit insufficient adhesiveness to polyolefin nonwoven fabrics such as envelopes.
To solve this problem, the use of a phthalate compound as the solid plasticizer (Japanese Patent Application Laid-Open (JP-A) No. 08-20668) and the use of a dihydric or polyhydric alcohol in combination with an ester compound comprising an aromatic monobasic acid (JP-A No. 08-253747) have been proposed, but these techniques are still insufficient.
The adhesive strength of the heat-sensitive adhesive material to an adherend decreases with time. As a possible solution to this problem, JP-A No. 09-169870 proposes the use of an ester compound containing a polyhydric alcohol and an aromatic monobasic acid as a crystallization retarder for inhibiting the crystallization of the solid plasticizer. However, this technique is still insufficient to solve the problem.
The heat-sensitive adhesive material cannot substantially be applied again to the adherend after initial application. To solve this problem, the adhesive strength of the adhesive layer of the heat-sensitive adhesive material should be somewhat low immediately after thermal activation of the adhesive layer, which allows the heat-sensitive adhesive material to be reapplied to the adherend, and increase with time to firmly stick to the adherend and to thereby prevent delamination. However, a heat-sensitive adhesive material having such adhesive properties has not yet been provided in practical use.